The present invention relates to the polarization induced signal fading of optical signals and particularly to an apparatus and method for minimizing the fading.
Mixing between a reference signal and a data signal is often necessary to extract information from an optical carrier. In communication, the mixing is typically between the received signal and a local oscillator signal at a different frequency. The result is an intermediate frequency (IF) that can be demodulated. In interferometric sensing, the mixing would be between a reference signal and a signal whose phase has been modified by the parameter being measured. The result is an interference signal. In both communication and interferometric sensing, amplitude of the mixed output is dependant upon efficiency of the mixing between the two input optical signals.
When two signals have the same state of polarization, their mixing is 100% efficient; when two signals have orthogonal polarization states, no mixing occurs. Between these two limits, only that portion of the signals whose states of polarization resolve onto a single axis undergo mixing. The reduction in the amplitude of the mixed signal due to an unmixed component in an orthogonal state of polarization is termed polarization induced fading.
The present art uses polarization diversity to overcome polarization induced fading in communication applications. A stable reference signal is equally divided between orthogonal axes. One method of accomplishing this is by aligning the state of polarization of a laser at 45.degree. to the orthogonal axis of a linear-polarization beam splitter. A random state of polarization input is resolved onto the orthogonal axes of the beam splitter. The signal is divided between horizontal and vertical channels, each of which contains equal amounts of reference light: the resolved signal components mix with the reference components and no signal fading occurs.
This solution, while overcoming the polarization induced fading, creates new problems. Because the signal is resolved onto the orthogonal axes as a function of the arbitrary input state of polarization while the reference signal is equally divided between the axes, the mixed signals from the two orthogonal axes do not sum to an optimum signal. Weighting or decision circuits are used to combine the signals from the two channels. Also, the arbitrary state of polarization of the input may align with one of the reference axes. If this occurs, the optical detector for the orthogonal channel will receive nothing but reference light: shot noise from this detector degrades the system signal-to-noise ratio.
In the case of applications to fiber interferometers, the state of polarization of the light in both fibers is arbitrary and varies over time. Therefore, the polarization diversity technique devised for communication applications, i.e., of equally dividing the reference light, cannot be used for interferometric sensor applications. Polarization masking provides some advantages: e.g., masking using axes at 0.degree., 60.degree., and -60.degree. assures that an interference signal is always present. However, because the measurement and reference lights are equally divided among the axes, the maximum output available is only one third of the input signal even if an individual axis is optimized.